Field of the Invention
The present invention relates to light emitting devices and, in particular, to the structure thereof.
Description of the Background Art
A light emitting device (LED) including a light emitting part made of a Group 13 nitride (Group III nitride) is already widely known. The previously known light emitting device usually includes a substrate (different-material substrate) made of a material, such as sapphire, different from the Group 13 nitride as an underlying substrate, and a plurality of crystalline layers made of the Group 13 nitride are stacked on the underlying substrate.
For example, a self-emitting display that includes a substrate at least having an insulating surface, lower wiring formed on the substrate, a silicon nitride film serving as a mask, and a plurality of columnar light emitting parts formed at positions of a plurality of openings of the mask, is already well known (see, for example, Japanese Patent Application Laid-open Publication No. 2013-55170). The substrate is, for example, a sapphire substrate, a ceramic substrate, and a silicon substrate on which a silicon oxide film has been formed. The lower wiring includes a silicon thin film and an n-type GaN layer. The plurality of columnar light emitting parts are formed by stacking a semiconductor layer of a first conductivity type, an active layer, and a semiconductor layer of a second conductivity type each made of the Group 13 nitride at positions of the plurality of openings. In the self-emitting display, a lower refractive index material having a lower refractive index than a semiconductor, which is a material for the light emitting parts, is located around the light emitting parts.
Technology for densely forming columnar LED structures made of the Group 13 nitride and referred to as nanocolumns as they each have a diameter of 1 μm or less on an n-type silicon monocrystalline substrate by RF-MBE is also already well known (see, for example, Akihiko Kikuchi, Mizue Kawai, Makoto Tada, and Katsumi Kishino, “InGaN/GaN Multiple Quantum Disk Nanocolumn Light-Emitting Diodes Grown on (111) Si Substrate”, Japanese Journal of Applied Physics, Vol. 43, No. 12A, 2004, pp. L1524-L1526).
Furthermore, technology for manufacturing an oriented polycrystalline substrate made of a compound semiconductor having an orientation in a c-axis direction and technology for forming a nitride compound semiconductor film on the substrate to stack a semiconductor layer including a light emitting layer are well known (see, for example, Japanese Patent No. 3410863).
On the other hand, a method for manufacturing a gallium nitride monocrystalline free-standing substrate by a flux method is also already well known (see, for example, WO2013/147326).
One factor that prevents the previously known light emitting device including the different-material substrate as the underlying substrate from having improved emission intensity is the difference in lattice constant and coefficient of thermal expansion between the underlying substrate and the Group 13 nitride layers. The difference causes propagation of dislocations in the Group 13 nitride layers constituting the light emitting parts, and current leakages occur at positions of the dislocations.
Use of the gallium nitride monocrystalline free-standing substrate as the underlying substrate solves the problem of the difference in lattice constant and coefficient of thermal expansion from the Group 13 nitride layers, but causes another problem of a rising device manufacturing cost as it is not easy to increase the area of the gallium nitride monocrystalline free-standing substrate.
The light emitting devices disclosed in Japanese Patent Application Laid-open Publication No. 2013-55170 and Kikuchi et al. include the different-material substrates, but have improved luminous efficiency by devising the structure thereof. Technology disclosed in Japanese Patent Application Laid-open Publication No. 2013-55170, however, has a problem in that a light emitting device having a vertical structure cannot be configured due to the use of an insulating substrate. Technology disclosed in Kikuchi et al. can be used to form light emitting parts with reduced dislocations and strain, but has cost restraints due to the use of the silicon monocrystalline substrate.